**Flexor Tendon Injuries**

**Flexor Tendon Injuries**

Justin Yousef and Sarah Anthony Justin Yousef and Sarah Anthony Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73392

#### **Abstract**

Even though flexor tendon injury is common, much remains to be elucidated about the best way to facilitate the intrinsic healing process and to minimise scarring. This chapter will comprise flexor tendon anatomy, types of flexor tendon injury, modes of healing, molecular updates, repair techniques and post-operative rehabilitation.

DOI: 10.5772/intechopen.73392

**Keywords:** anatomy, flexor tendon injury, rehabilitation, tendon healing, molecular updates, complications

**1. Introduction**

Repair of the injured flexor tendon in the hand to achieve normal function remains a difficult task and is made even more so with complications such as repair rupture and adhesions. Controversy exists as to what post-operative rehabilitation protocols should be employed and is often based on surgeon experience or the hand therapy service. This chapter aims to focus on postoperative rehabilitation and future research trends. The reader is encouraged to seek alternative resources for more detail regarding flexor tendon anatomy, pathophysiology and repair techniques as these will be covered briefly in this chapter.

### **2. Flexor tendon anatomy**

#### **2.1. Flexor digitorum superficialis**

Flexor digitorum superficialis (FDS) is responsible for flexion of the proximal interphalangeal (PIP) joint as well as contributing to the flexion of the metacarpophalangeal (MCP) joints. The muscle body has two heads of origin. The ulnar head arises from the anterior aspect of the

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

medial epicondyle, the ulnar collateral ligament of elbow, medial aspect of coronoid process, and the proximal ulna. The radial head originates from the proximal radius immediately distal to the insertion of supinator. At the mid-forearm, the FDS muscle belly divides into the superficial and deep layers. The superficial layer gives a tendon to the middle finger and ring finger, whilst the deep layer supplies the index and little fingers. Each tendon arises from a separate muscle belly allowing independent flexion of PIPJ of each finger except for LF which may have cross connection with FDS tendon of RF [1] as the FDS to the little finger may be hypoplastic or absent in some people [1].

certain areas called pulleys. The pulleys are either annular (A) or Cruciform (C) in shape and are numbered proximal to distal. The odd numbered pulleys A1, A3, A5 are over the MCP, PIP and DIP joints respectively [1, 3]. The A2 pulley overlies the proximal phalanx and the A4

Flexor Tendon Injuries

23

http://dx.doi.org/10.5772/intechopen.73392

The cruciform pulleys are located proximal and distal to the PIP joints. Between the A2 and A3 pulleys is C1, between the A3 and A4 pulleys is C2, and the C3 pulley is located just proximal to the DIP joint between the A4 and A5 pulleys. The cruciform pulleys, in conjunction with A1, A3 and 5 permit compression of the flexor tendon without impingement during flexion and expansion of the tendon during extension [4]. The A2 and A4 pulleys have traditionally been deemed most important in preventing bowstringing and therefore most surgeons advocate their preservation, repair or reconstruction [4]. However, it has been demonstrated that partial distal excisions of 25% of the A2 pulley, up to 75% of the A4 pulley and 25% of combined A2 and A4 have no significant effect on digit range of motion or work of flexion [5, 6].

Tendons are collagen-based tissues that connect muscle to bone. They are composed primarily of Type I collagen, whereas the surrounding endotenon and epitenon are composed of Type III collagen [7]. The collagen itself is synthesised and secreted by tenocytes within the tendon and, once secreted, the collagen fibres arrange into triple helices to increase strength and stability.

Collagen fibre units are bound together by endotenon fascicles. These fascicles are bound in turn by the epitenon to form the whole tendon. Lymphatic, vascular and neural constituents are present within the endotenon to supply the fibroblasts whereas the epitenon contains the

The tendon sheath is covered with synovial cells that lubricate the tendon to help gliding within the sheath. Outside of the hand, tendons are not typically enclosed within a sheath but are covered by paratenon that contains the vascular elements to supply both the endotenon and epitenon.

Both FDP and FDS tendons in the digits receive dual nutritional supply from vascular perfusion and synovial diffusion [7]. Vascular perfusion is provided by the vincula with each tendon having two: a vinculum longus and a vincula brevis. From proximal to distal, the first vinculum encountered is the vinculum longus superficialis (VLS), arising just proximal to Camper's chiasm and coming off the floor of the digital sheath of the proximal phalanx The vinculum brevis superficialis consists of small triangular mesenteries near the insertion of the FDS. The vinculum longus profundus arises from the superficialis at the level of the PIP joint. The vinculum brevis profundus arises near the insertion of the FDP. Each vinculum inserts on the dorsal aspect of the tendon, creating a richer blood supply on the dorsal side of the tendon. The vincula are important in the repair of injured tendons as they may hold the tendons out to length after injury. Therefore, one must be careful not to injure any maintained vincula while repairing an injured tendon, thereby decreasing the already tenuous blood supplyThe parietal paratenon allows for passive nutrition delivery to, and waste removal from, the tendons within the sheath by diffusion [8]. Nutrient supply is also directly from the bones of the

pulley over the middle phalanx. The even numbered pulleys.

**2.5. Tendon anatomy and nutrient supply**

blood vessels and tracts for the lymphatics and nerves.

fingers from the middle (FDS) and distal (FDP) phalanges.

At level of the proximal phalanx, FDS divides into two slips which pass on either side of the tendon of flexor digitorum profundus (FDP) forming "Camper's Chiasm" [1] and then unites to insert onto the volar surface of the middle phalanx. The blood supply to FDS is from both ulnar and radial arteries and its innervation is from the median Nerve which enters the muscle belly in proximal forearm.

#### **2.2. Flexor digitorum profundus**

Flexor digitorum profundus (FDP) is responsible for flexion of the distal interphalangeal (DIP) joints of the fingers as well as contributing to flexion of the PIP and MCP joints. It arises from volar and medial aspects of the proximal three quarters of the ulna and from the medial half of volar aspect of the interosseous membrane. It lies deep to FDS and adjacent to flexor pollicis longus (FPL) in the forearm. The Ulnar nerve travels distally in the forearm, loosely adhered to FDP, innervating the muscle-tendon units for the ring Finger and little fingers [1]. The anterior interosseous nerve (AIN) of the median nerve innervates the index and middle finger units.

Unlike FDS, the tendons of FDP usually arise from common muscle belly. Frequently however. The tendon to the index Finger arises from a separate muscle belly resulting in greater independence of flexion [1].

#### **2.3. Flexor pollicis longus**

Flexor pollicis longus (FPL) is responsible for flexion of the interphalangeal (IP) joint of the thumb. It arises from volar aspect of the middle one third of the radial shaft and from the interosseous membrane. It is located deep to the flexor carpi radials tendon and the radial artery at the level of the proximal wrist crease. FPL inserts onto the base of the distal phalanx of the thumb. The AIN innervates FPL in the proximal forearm and the blood supply is predominantly from the radial artery. All nine digital flexors enter the carpal tunnel with the four FDP tendons deep to FDS tendons of the little finger and index finger. FDS tendons of ring finger and middle finger lie superficially immediately ulnar to median nerve. The tendon of FPL lies deep and radial within the carpal tunnel, adjacent to scaphoid and trapezium.

#### **2.4. Flexor sheath and pulleys**

Proximal to MCP joints the flexor tendons enter the fibro-osseous flexor sheath. This tunnel functions to hold the tendons in close proximity to the phalanges to prevent "bowstringing" and to increase the efficiency of tendon glide [2]. The flexors heath tunnel is thickened in certain areas called pulleys. The pulleys are either annular (A) or Cruciform (C) in shape and are numbered proximal to distal. The odd numbered pulleys A1, A3, A5 are over the MCP, PIP and DIP joints respectively [1, 3]. The A2 pulley overlies the proximal phalanx and the A4 pulley over the middle phalanx. The even numbered pulleys.

The cruciform pulleys are located proximal and distal to the PIP joints. Between the A2 and A3 pulleys is C1, between the A3 and A4 pulleys is C2, and the C3 pulley is located just proximal to the DIP joint between the A4 and A5 pulleys. The cruciform pulleys, in conjunction with A1, A3 and 5 permit compression of the flexor tendon without impingement during flexion and expansion of the tendon during extension [4]. The A2 and A4 pulleys have traditionally been deemed most important in preventing bowstringing and therefore most surgeons advocate their preservation, repair or reconstruction [4]. However, it has been demonstrated that partial distal excisions of 25% of the A2 pulley, up to 75% of the A4 pulley and 25% of combined A2 and A4 have no significant effect on digit range of motion or work of flexion [5, 6].

#### **2.5. Tendon anatomy and nutrient supply**

medial epicondyle, the ulnar collateral ligament of elbow, medial aspect of coronoid process, and the proximal ulna. The radial head originates from the proximal radius immediately distal to the insertion of supinator. At the mid-forearm, the FDS muscle belly divides into the superficial and deep layers. The superficial layer gives a tendon to the middle finger and ring finger, whilst the deep layer supplies the index and little fingers. Each tendon arises from a separate muscle belly allowing independent flexion of PIPJ of each finger except for LF which may have cross connection with FDS tendon of RF [1] as the FDS to the little finger may be

At level of the proximal phalanx, FDS divides into two slips which pass on either side of the tendon of flexor digitorum profundus (FDP) forming "Camper's Chiasm" [1] and then unites to insert onto the volar surface of the middle phalanx. The blood supply to FDS is from both ulnar and radial arteries and its innervation is from the median Nerve which enters the

Flexor digitorum profundus (FDP) is responsible for flexion of the distal interphalangeal (DIP) joints of the fingers as well as contributing to flexion of the PIP and MCP joints. It arises from volar and medial aspects of the proximal three quarters of the ulna and from the medial half of volar aspect of the interosseous membrane. It lies deep to FDS and adjacent to flexor pollicis longus (FPL) in the forearm. The Ulnar nerve travels distally in the forearm, loosely adhered to FDP, innervating the muscle-tendon units for the ring Finger and little fingers [1]. The anterior interosseous nerve (AIN) of the median nerve innervates the index and middle finger units. Unlike FDS, the tendons of FDP usually arise from common muscle belly. Frequently however. The tendon to the index Finger arises from a separate muscle belly resulting in greater

Flexor pollicis longus (FPL) is responsible for flexion of the interphalangeal (IP) joint of the thumb. It arises from volar aspect of the middle one third of the radial shaft and from the interosseous membrane. It is located deep to the flexor carpi radials tendon and the radial artery at the level of the proximal wrist crease. FPL inserts onto the base of the distal phalanx of the thumb. The AIN innervates FPL in the proximal forearm and the blood supply is predominantly from the radial artery. All nine digital flexors enter the carpal tunnel with the four FDP tendons deep to FDS tendons of the little finger and index finger. FDS tendons of ring finger and middle finger lie superficially immediately ulnar to median nerve. The tendon of FPL lies deep and radial within the carpal tunnel, adjacent to scaphoid and trapezium.

Proximal to MCP joints the flexor tendons enter the fibro-osseous flexor sheath. This tunnel functions to hold the tendons in close proximity to the phalanges to prevent "bowstringing" and to increase the efficiency of tendon glide [2]. The flexors heath tunnel is thickened in

hypoplastic or absent in some people [1].

muscle belly in proximal forearm.

22 Essentials of Hand Surgery

**2.2. Flexor digitorum profundus**

independence of flexion [1].

**2.3. Flexor pollicis longus**

**2.4. Flexor sheath and pulleys**

Tendons are collagen-based tissues that connect muscle to bone. They are composed primarily of Type I collagen, whereas the surrounding endotenon and epitenon are composed of Type III collagen [7]. The collagen itself is synthesised and secreted by tenocytes within the tendon and, once secreted, the collagen fibres arrange into triple helices to increase strength and stability.

Collagen fibre units are bound together by endotenon fascicles. These fascicles are bound in turn by the epitenon to form the whole tendon. Lymphatic, vascular and neural constituents are present within the endotenon to supply the fibroblasts whereas the epitenon contains the blood vessels and tracts for the lymphatics and nerves.

The tendon sheath is covered with synovial cells that lubricate the tendon to help gliding within the sheath. Outside of the hand, tendons are not typically enclosed within a sheath but are covered by paratenon that contains the vascular elements to supply both the endotenon and epitenon.

Both FDP and FDS tendons in the digits receive dual nutritional supply from vascular perfusion and synovial diffusion [7]. Vascular perfusion is provided by the vincula with each tendon having two: a vinculum longus and a vincula brevis. From proximal to distal, the first vinculum encountered is the vinculum longus superficialis (VLS), arising just proximal to Camper's chiasm and coming off the floor of the digital sheath of the proximal phalanx The vinculum brevis superficialis consists of small triangular mesenteries near the insertion of the FDS. The vinculum longus profundus arises from the superficialis at the level of the PIP joint. The vinculum brevis profundus arises near the insertion of the FDP. Each vinculum inserts on the dorsal aspect of the tendon, creating a richer blood supply on the dorsal side of the tendon. The vincula are important in the repair of injured tendons as they may hold the tendons out to length after injury. Therefore, one must be careful not to injure any maintained vincula while repairing an injured tendon, thereby decreasing the already tenuous blood supplyThe parietal paratenon allows for passive nutrition delivery to, and waste removal from, the tendons within the sheath by diffusion [8]. Nutrient supply is also directly from the bones of the fingers from the middle (FDS) and distal (FDP) phalanges.

#### **2.6. Flexor zones**

The volar surface of the hand is divided into five anatomic zones from distal to proximal. When describing a zone for an injury it is referred to by the zone that it would lie in the hand in the resting posture.

**Zone 1** commences at the fingertip and ends at the insertion of FDS on the middle phalanx. It includes the C3 and A5 Pulleys and the FDP tendon.

**Zone 2** is from the insertion of the FDS to the proximal aspect of the A1 pulley. It contains both FDS and FDP.

**Zone 3** is from the proximal A1 pulley to the distal limit of the transverse carpal ligament (TCL).

**Zone 4** is the area deep to the TCL i.e. the carpal tunnel.

**Zone 5** is from the proximal edge of the TCL to the musculotendinous junction.
